Fractional distillation apparatus



Sept. 11-, 1962 w. E. PLASTER ETAL FRACTIONAL DISTILLAToN APPARATUS 2Sheets-Sheet 1 Filed Oct. l, 1959 INVENTORS W. E. PLASTER R.E. DIXONATTORNEYS R E L L o R T N O C mm D RI- mm R Rm mm 00, u E .c lw. mw A TOIrv MHS@ E L MU www n m T L m .@A 2|v n ai r7- A w TL' w H 4 .0.1km H I2 H Q H 1J :LH .I/(l 3 a n, m m Y m u 3,053,521 FRACTIONAL DISTILLATIONAPPARATUS William E. Plaster and Rolland E. Dixon, Bartlesville,

Okla., assignors to Phillips Petroleum Company, a corporation ofDelaware Filed Oct. 1, 1959, Ser. No. 843,784 12 Claims. (Cl. 261--114)This invention relates to fractional distillation. In one aspect itrelates to a fractional distillation column with provision for removalof a side stream. In another aspect it relates to a weir assembly for aside draw tray for use in a fractional distillation column.

In the control of a fractional distillation column, particularly when amajority of the internal reflux is withdrawn as product from a side drawtray of the column, it is important to make certain that at least someliquid ows downward to the next tray in order that that portion of thecolumn Iwill not run dry. According to our invention, we have devised aside draw tray -for a fractional distillation column in which provisionis made that there will always be at least some liquid flowing downwardlfrom the take-off tray to the next lower tray. Our tray is notordinarily provided with vapor-liquid contact promoting apparatusbecause it is mainly a take-olf tray, not a contacting tray. This trayis provided with a weir in which openings are made through which liquidas reflux flows to the next lower tray. It is intended that all of `theliquid reflux flo-wing downward from this tray pass through openings sothat its volume can be determined. Preferably, it is intended thatdownowing liquid shall not pass over the top of the Weir as in normalfractional distillation trays. One system is to drill holes or openingsof a predetermined size or conguration through which liquid can ow tothe next lower tray. It is intended that the combined area of the holesor openings is such that all of the liquid at normal flow, or at a owslightly greater than normal, will pass through the holes rather 'thanover the top of the weir. In this manner the rate of ilow of thedownflowing liquid can be quite accurately determined. At least one ofthe above-mentioned openings is positioned near the bottom ofthe weirplate so that as long as there is liquid onV the take-olf tray some of-it will ilow downward as reflux to prevent the column going dry at thispoint. At higher elevations on the weir plate the number of openings, orrather the opening area, is greater than near the bottom of the plate.Other coniiguration than holes or openings can be used as, for example,a V-shaped notch, with the apex of the V pointing downward andpositioned substantially at the deck of the tray. A Weir with holes oropenings as mentioned above is preferred because such a weir possessesgreater strength and requires less bracing to prevent buckling. Othershapes of openings can be used in making the weir plate, but the twotypes of openings mentioned are preferred because they are more easilymade.

A liquid level indicator can be used in conjunction with the tray toindicate the level of liquid thereon. Circular Weir openings andV-shaped weir openings are furthermore preferred because orificeformulas are available for calculating the flow through such weirs forvarious heights of liquid above the bottom of the weir. By using a levelindicator and a proper oriiice formula, the flow of liquid through theweir is easily determined and this flow of liquid, of course, is theinternal reilux of the column flowing downward from the take-off tray ofthis invention.

Furthermore, we also provide a liquid level controller and a reset flowcontroller for control of a motor valve in the take-olf line from ourside draw tray. This apparatus is provided so that the flow controllerwith a pneumatically operable set point controls. the rate of liquid A-PatenteclSepL 11, 1962V withdrawal from the tray and the liquid levelcontroller resets the set point of the flow controller in such a man-Aner that at higher liquid levels on the tray the rate of withdrawal ofthe side stream is increased.

An object of this invention is to provide a side draw tray for use in afractional distillation column for withdrawal of a side stream productand at the same time make provision for passage of retlux to a nextlower and vapor-liquid contacting tray. Another object of this inventionis to provide such a side draw tray on which the level of the liquid iseasily determined. Still another object of this invention is to providesuch a tray and apparatus therefor -for determining the rate of passageof reflux to aV next lower tray. Still other objects and advantages ofthis invention will be realized upon reading the following descriptionwhich, taken with the attached drawing, forms a part of thisspecification.

In the drawing,

FIGURE l is an elevational View, partly in section, of one embodiment ofside draw trayof this invention.

FIGURE 2 is a sectional view taken on the line 2P-,2 of FIGURE l. I

FIGURE 3 is an elevational view, partly in section, of anotherembodiment of our invention.

FIGURE 4 is a sectional View taken on the line 4,-4

of FIGURE 3. j Y

FIGURE 5 is a view taken from the line 5-5 of FIG- URE 3.

FIGURE 6 is an elevational view, partly in section, of anotherembodiment of our invention and taken on the line 6--6 of FIGURE 7.

FIGURE 7 is a sectional View taken on the line 7-7 of FIGURE 6.

FIGURE 8 is an elevational view taken on the line 8-8 of FIGURE 6.

FIGURE 9 is an elevational View of an alternate embodiment of theapparatus of FIGURE 8.

In the drawing reference numeral 11 identities `a fractionaldistillation column provided with conventional bubble cap trays 12 forpromoting vapor-liquid contacting. A side stream product take-off tray13 comprises a deck 13a containing one or more chimneys 14 and -aperforate baffle plate `or Weir 15. This perforate Weir 15 is providedwith perforations 15a. A plate l16 is provided to define adowncomer space `between that plate and the walls of the column 11 fordownward passage of liquid from the tray above to the take-off tray.The? lower end of baille 16 is near the deck 13a so `as to provide avapor trap. The lower end of the perforate baffle or fweir 15 yalsoextends downward to a point near the next lower tray `for providing avapor seal. The portion of the baille plate 15 below deck 13a is notperforated. A pipe 17 leads -from the level of the top surface or deck13a for withdrawal of the liquid side stream product. Pipe 17 isprovided with ta throttle valve 22 and an orifice plate assembly 21.This valve and orifice plate assembly communicate with a pneumaticallyoperable reset flow controller 20 for controlling the rate oli/flowthrough pipe `17. A liquid level lcontroller 19 is provided inconjunction with the liquid level sensing means 18 for indi cating thelevel of the liquid lon the take-off tray 13. These instruments are soinstalled and regulated that upon increase of liquid level on tray 13a,the liquid level controller l1-9 resets the set point of flow controller20 to increaseV the opening of the throttle valve 22. In other Words, asthe level of the liquid on tray 13a rises, the rate of withdrawal ofside product is increased; and con--4 versely, when the level of liquidon the take-ofi:` tray 13a drops, throttle valve 22 is throttled toreduce the rate of product removal. p

The controller 19 can, if desired, be a recorder-con` troller. By usinga liquid level krecorder-controller, a

permanent record of the level of the liquid on tray 13a is obtained andthis level controller resets the set point of llow controller 20 toregulate the rate of liquid removal. Since the area of the Weir openingor openings 15a in the perforate Weir 15 is greater at higher levels ofthe weir, as the liquid level on tray 13a rises the flow of reflux tothe next lower tray increases. Conversely, at low liquid levels on tray13a, the flow of liquid through perforations 15a is smaller. FIGURE 8illustrates an arrangenient of circular openings through a baflle plate,similar to plate 15, suitable for use according to this invention. Theliquid level recorder-controller can be calibrated to record the `llowof liquid through the perforations 15a in response to the level of theliquid on the tray. Such calibration makes use of an equation such asQ=cA\/2gh for each opening in the plate, and in which Q equals gallonsof llow per minute, c -is the orice coeicient, A is the area of theopening in square feet, h is the feet of head of liquid above the centerof the orifice, and g equals 32.2 ft./sec.2

In FIGURE 3 is illustrated an alternate embodiment of our invention inwhich a fractionating column 23 is provided with conventional bubble captrays 24 provided with vbubble caps 53. In this embodiment a side drawproduct take-off tray 25 is a combination side product take-off tray andvapor liquid contacting tray. In this case vapor flows upward throughthe take-off rtray by way of bubble caps 53. A low Weir 26 is providedfor maintaining at least a minimum liquid level on the tray 25 forvapor-liquid contacting. A well 51, defined by a side plate 29, a floorplate 30, another side plate 27 and the walls of column 23 is providedfor maintenance of a quiescent volume of liquid. This quiescent volumeof liquid is necessary for proper operation of a liquid level indicatingfloat. The baille 27 is the perforate baille according to thisinvention, the perforations being identied by reference numeral 38. Apair of openings 28 is provided for communication with a float chamber37, illustrated in FIGURE 5. The float within float chamber 37 isconnected with `a liquid level recorder-controller 32 which, in turn, isconnected with -a reset flow controller 33 for resetting the set pointthereof for regulation of a throttle valve 35 to control the rate ofliquid withdrawal from the well 51. Throttle valve 35 is positioned inpipe 31 along with orice plate `assembly 34. 'I'his control apparatus oflFIGURE operates in a manner similar to the operation of the apparatusof FIGURE 1. FIG- URE 3 illustrates the position of the float chamber 37(openings 28) relative to the well 51. Plate 36 is the downcomer platefrom the vbubble cap tray above and maintains a vapor seal on thetake-ofir tray 25.

The apparatus of FIGURE 6 is intended as large diam eter equipment inwhich a fractionating column 39 is a dual-flow column in that there aretwo parallel streams of reflux flowing downward in the columncountercurrent with respective upflowing vapors. Such columns are commonin the petroleum refining art. In this figure, takeoff tray 41 isprovided with a well 52 common fto both sides of the column. At one endof well 52 is an opening 45 to which is connected the liquid take-olfwithdrawal pipe, not shown. Plates 44 extend downward from theimmediately above -bu-bble cap tray to provide vapor seals. Chimneys 42permit passage of vapor through the takeoff tray 41. Perforate weirs 43contain openings 43a through which liquid flows downward to the nextlower bubble cap tray. `Openings 46 in the wall of column 39 lare forattachment of a liquid llevel indicating float chamber 50, illustratedin FIGURE 7. FIGURE 8 is a side view of perforate weirs 43 illustratingthe positioning of the openings 47. In this figure it is noted that theopenings at levels near the surface of the take-off tray 41 are singleopenings. If desired, these openings may be made progressively smallerat elevations near the tray 41. At levels somewhat above the tray thereare illustrated pairs of openings while at a still higher elevation areillustrated three openings at the same level. In the combined area ofthese several openings in such a perforate Weir `as weir plate 43, it isintended that all of the downilow reflux to the next lower vapor-liquidcontacting tray at normal or slightly greater than normal flow ratesflows through these openings 47 with none llowing over the top of theWeir plate. By applying formulas such as that mentioned hereinabove, andusing openings such as openings 47, the reflux flow can be determinedand recorded quite accurately.

In FIGURE 9 is illustrated an alternate type of Weir plate. This plate49 is provided `with a V-shaped weir 48 with the apex of the V pointingdownward and at an elevation near the top surface of the take-off plate.A suitable formula for liquid tlow through such a Weir opening is usedfor calibration of a liquid level recorder-controller so that at variousheights of level of liquid above the apex of the V the rate of refluxllow is easily determined.

An example of the operation of a take-ofi tray assembly of thisinvention is illustrated in the following tabulation. These data are interms of barrels per stream day to and from a depropanizer columnproducing a propane- `rich overhead product, an isobutane-rich sidestream and a bottoms product comprising mainly pentanes and higherboiling hydrocarbons along with an appreciable proportion of normalbutane. Such a column is used in a hydrolluoric acid alkylation system.The 5,767 barrels per stream day of isobutane reflux is that refluxllowing through a perforate orifice plate according to this invention toa next lower bubble cap tray. This 5,767 barrels per stream day isequivalent to 210 gallons per minute at tray conditions. This columntreats 20,664 barrels of feed per day with the production of the severalproducts as indicated in the tabulation.

Isobutane Reflux Feed Bottoms Sidedi'aw Net Overhead Pen tanes-l- Total20, 647 3, 644

is, 10i

1 5,767 bbls. per stream day equals 210 gallons per minute at trayconditions.

50 c I n a a It will be noted that by positioning at least a portion ofthe weir opening at a level near the upper surface of the take-olf tray,there is always liquid reflux flowing to a next lower vapor-liquidcontacting tray. Thus, by so positioning the weirs of this invention,none of the coritacting trays below the takeot tray ever go dry. It isrealized -by those skilled in the art that when a vaporliquid contactingtray in a fractional distillation column goes dry there is a seriouscolumn upset and frequently considerable periods of time are required toright the wrong done by such a column upset.

Materials of construction may be selected from those commerciallyavailable, taking into consideration the corrosive nature of thematerials to be fractionated.

While certain embodiments ofthe invention have been described forillustrative purpos, the invention obviously is not limited thereto.

We claim:

1. A side draw tray section for a fractional distillation columncomprising, in operable combination, a vertical section of a fractionaldistillation column, a tray, a portion of the periphery of said trayfitting said section 0f column and the remainder being a chord joiningthe ends of said portion of periphery, the area of said tray being 75such that the tray occupies a major portion of the crosssectional areaof said section of column, a plate attached to the edge of said traydened by said chord, said plate extending upward and extending downwardfrom the respective surfaces of said tray, an unobstructed vapor chimneyin said tray extending to a level above the top edge of said plate, arst opening in said plate above and adjacent the top surface of saidtray, a second opening in the wall of said section of column adjacentand above said tray for withdrawal of a side product, a liquid levelsensing means in operative communication with the space above said trayand below the top edge of said plate, and means to indicate the rate ofliquid ow through said first opening in response to the level of liquidon said tray as sensed Iby said liquid level sensing means.

2. In the side draw tray section of claim l wherein said iirst openingis a perforation.

3. In the side draw tray section of claim l wherein said iirst openingis a V-shaped slot with the apex of the V pointing downward.

4. A fractional distillation column comprising, in operable combination,a tubular, upright column, a tray positioned across the cross section ofsaid column at a level at which a side product is to be withdrawn, aportion of the periphery of said tray being an arc and supported fluidtight by the inner surface of said column, and the remainder being achord joining the ends of said arc, a vertically disposed platepositioned along the edge of said tray defined by said chord, said plateextending upward and downward from said tray, the space between saidplate and the adjacent section of wall of said column defining adowncomer, a lirst opening in said plate at a level above and near thetop surface of said tray, vaporliquid contact promoting apparatuspositioned operatively at levels above and below said tray, a vaporchimney in said tray and extending to a level above the top edge of saidplate, means for entry of lfeed material into said column, a vaporchimney in said tray extending to a level above the top` edge of saidplate, means for entry of feed material into said column, separate meansfor outlet of overhead vapor and bottom materials from said column, asecond opening in the Wall of said column adjacent the top surface ofsaid tray for outlet of a side draw product, a conduit communicatingwith said second opening, a motor valve in said conduit, a liquid levelsensing means in communication with the space immediately above the topsurface of said tray, a liquid level indicating-controller communicatingoperatively with sai-d liquid level sensing means, a reset ilowcontroller in operative communication with said motor valve and withsaid indicatingcontroller, the liquid level indicator of said controllerbeing calibrated to indicate liquid flow through said first opening asreliux in response to level of liquid on said tray as sensed by saidliquid level sensing means, and whereby upon sensing a level of liquidon said tray said controller regulates said motor valve therebycontrolling flow of liquid therethrough with the simultaneous regulationof flow of liquid through said first opening as reilux.

5. In the column of claim 4 wherein said rst opening is a perforation.

6. In the column of claim 4 wherein said first opening is a V-shapedslot with the apex of the V pointing downward.

v 7. A side draw tray section for a fractional distillation columncomprising, in operable combination, a vertical section of a fractionaldistillation column, a tray, a major portion of the periphery of saidtray fitting the inner wall of said section of column, the remainder ofsaid periphery being so shaped as to be positioned at a spaced distance,from the adjacent inner wall of Said column thereby Providing a freeedge, a plate attached to said free edge, said plate extending upwardand extending downward from said tray, an unobstructed vapor ilowchimney in said tray extending to a level above the top edge of saidplate, a first opening in said plate above the top surface of said tray,a second opening in the wall of said section of column adjacent andabove said tray for withdrawal of a side product, a liquid level sensingmeans in operative communication with the space above said tray andbelow the top edge of said plate, and means to indicate the rate ofliquid flow through said rst opening in response to the level of liquidon said tray as sensed by said liquid level sensing means.

8. In the side draw tray section of claim 7 wherein said first openingis a perforation.

9. In the side draw tray section of claim 7 wherein said rst opening isa V-shaped slot with the apex of the V pointing downward.

l0. A fractional distillation .column comprising, in operablecombination, an upright column, a tray positioned across the column at alevel at which a side product is to -be withdrawn, a major portion ofthe periphery of said tray itting the inner wall of said column, theremainder of the periphery being a spaced distance from the inner wallof said column thereby providing a free edge, a plate attached at oneside surface to said free edge, said plate extending upward to a levelabove and to a level below said tray, the space between said plate andthe adjacent wall of said column defining a downcomer, a lirst openingin said plate above and near the top surface of said tray, vapor-liquidcontacting apparatus positioned operatively at levels above and belowsaid tray, a vapor chimney in said tray extending to a level above thetop edge of said plate, means for introducing feed stock into saidcolumn, separate means for outlet of overhead vapor and bottoms materialfrom said column, a second opening in the wall of said column above andadjacent the top surface of said tray for outlet of a side draw product,a conduit exterior of said column communicating with said secondopening, a motor valve in said conduit, a liquid level sensing means incommunication with the normally liquid containing space on said tray, arecorder-controller communicating operatively with said liquid levelsensing means and with said motor valve, the recorder ofrecordercontroller fbeing calibrated to record liquid flow through saidfirst opening as reilux in response to level of liquid on said tray assensed by said sensing means, and whereby upon sensing a predeterminedlow level of liquid on said tray said controller closes said valvethereby permitting liquid remaining on said tray to 'ow through saidiirst opening as reux as long as liquid reaches said tray from saidcontacting apparatus thereabove.

11. In the column of claim wherein said `iirst opening is a perforation.

l2. In the column of claim l0 wherein the first opening is a V-shapedslot with the apex of the V pointing downward.

References Cited in the file of this patent UNITED STATES PATENTS2,006,986 de Florez July 2, 1935 2,467,951 Whitley Apr. 19, 1942 OTHERREFERENCES Instruments and Process Control, information Sheet 4, pagesll to 17 and Infomation Sheet 12, pages 155 to 185, published by NewYork State Vocational and Practical Arts Assn., 1945,

